In the case of a standard wind turbine, the nacelle can carry three blades (approximately 60 m in length for example) fastened to a rotor that drives, through the agency of a gear reducer, an electric generator and accessories such as the blade orientation system, electric transformers, a hydraulic system, the ventilation. The structure of the nacelle rests on a slewing ring itself supported by the tower.
The nacelle is permanently oriented on the azimuth axis so that the rotation plane of the blades faces the wind. The nacelle is therefore mobile in rotation with respect to the axis of the tower, by means of one (or more) motor-driven gears cooperating with a ring gear. The overall weight of a nacelle can be of the order of 200 to 300 tons.
The tower supporting the nacelle, approximately 100 m in height, rests on a floating support held in position and substantially stabilized by tendons in case of water depths greater than 100 m.
The nacelle of the wind turbine undergoes many forces and stresses related to the wind speed, which results in a thrust on the blades supported by a thrust bearing whose mobile part is secured to the rotor shaft.
The value of this thrust power, or axial force, oriented in the wind direction, depends on the angle (pitch) of each blade with respect to the wind direction.
In the case of a wind turbine on a floating support, the floating support undergoes various forces due to the combined effects of the wind, the waves, the ocean currents, the tendons, the effects of the tower on the floating support, its tilt and flexion by the applied forces due to the wind and the nacelle.
The “wind turbine on floating support” system undergoes a set of complex forces acting in different directions, having different values, static, periodic, aperiodic. These forces, linked and interdependent, result from the effects of the wind on the blades and the nacelle, the tower, the floating support, the tendons and from the effects of the wind, the waves and the currents on this floating support.
It is the wind direction that imposes the direction of the axial force applied onto the rotor since the rotation plane of the blades must be permanently oriented perpendicular to the wind direction in order to obtain maximum wind energy absorption for a higher efficiency.
The wind direction in space can be defined by two angles:—the tilt angle of the wind direction with respect to the horizontal plane taken as the reference and—the azimuth angle formed, in the same horizontal plane, by the wind direction with the direction of the Earth's magnetic field taken as the reference.
The conventional wind turbines currently used onshore or offshore on a fixed or floating support comprise a nacelle rotationally mobile about the tower axis that can be oriented on the azimuth axis. However, no wind turbine comprises active means for controlling the nacelle tilt with respect to the horizontal reference depending on the periodic displacements due to the buoyancy of the support, or on the wind direction variations.